Ultra-thin glass: the way forward for the low-energy greenhouse?

Glass just 0.55 mm thick has been around for some time now. It’s on every mobile phone and tablet. It’s extremely light, strong and flexible – almost like plastic film. Could this be the material for the low-energy greenhouse of the future? What potential does it have? And for which crops? Two Dutch universities, Wageningen University & Research (WUR) and Delft University of Technology, have been working with various market parties to find out.

Every innovation starts with a far-fetched idea. Twelve years ago, no one thought that diffuse glass and glass with an antireflection (AR) coating would ever succeed when WUR first started trialling them. Likewise, ultra-thin glass won’t be ready for practical use from one day to the next. The price is still too high and the dimensions are too small. But the research was more than worthwhile, says Silke Hemming, head of WUR’s Greenhouse Technology research team.

Highly insulating

“I saw this material at a coatings factory once. It was so flexible I thought it was plastic. A while later, a Japanese glass manufacturer approached us with the same material. That’s when I started wondering whether it might have potential for commercial greenhouses.” Double glazing, as in the VenLowEnergy greenhouse, makes for a highly insulating roof but is heavy. Could ultra-thin glass be a lighter alternative? Could it also be an option for greenhouses with polycarbonate sheets? Just like multi-layer material, this glass is highly transparent. The second question is whether this flexible glass could lead to new shapes of greenhouse roofs.

And so the “Feasibility study thin glasses for greenhouse roof designs” was born. The study was commissioned by Kas als Energiebron, LTO Glaskracht Nederland and the Ministry of Agriculture, Nature and Food Quality and was partnered by glass manufacturers Asahi Glass Company (AGC), glass suppliers and surface treatment specialists Glascom Tuinbouw and greenhouse roof specialists Boal Systems. Delft University of Technology came on board at a later stage to test the mechanical properties.

Good light transmittance

First the research team tested the optical properties of Leoflex and Falcon glass, both from AGC. The glass was supplied in various thicknesses, both with and without AR treatment. The tests were carried out in the WUR LightLab.

AR treatment has been shown to increase transmittance by 7% and is therefore a requirement for the use of this material. The hemispherical light transmittance of 0.55 mm glass with AR (86.5%) is slightly higher than that of conventional 4 mm glass with AR (86.2%). Double, triple and quadruple layer versions of this thin glass sacrifice a little transmittance, but not much: triple glass with an AR treatment still clocks up 80.2% light transmittance. As a comparison, polycarbonate sheets have hemispherical transmittance of 63.5% – a lot lower.

Naturally, the partners in the research also attempted to make the material diffuse. Although they ultimately managed on a small area, it was tricky. “This glass is chemically tempered rather than thermally,” the project manager explains. “You can make it diffuse but the procedure is different, so it will involve more investment in the future. We only took measurements on small samples of glass.”

More layers, better insulation

The team also tested the thermal and mechanical properties of ultra-thin glass. The outcome of the first part came as no surprise: the more layers of glass there are, the better the insulation value. The researchers achieved an even lower U-value – the measure of how effective an insulator the glass is – by filling the air gap with krypton.

As far as the mechanical properties are concerned, the light weight is the most striking aspect. The extra weight that multi-layer glass adds to a greenhouse roof simply isn’t there with ultra-lightweight glass: 0.55 mm glass weighs just 1.4 kg per m2, compared with conventional 4 mm glass which weighs 10 kg per m2. That’s seven times as much. Hemming: “It’s therefore quite possible to use triple and quadruple layer ultra-thin glass. But there are other downsides. Up to now, the maximum available width has been 1200 mm, whereas the standard size needed for greenhouses is 1670 mm. What’s more, the fact that the glass is so flexible is a challenge. To be able to fit it you have to stiffen it by cold-bending or pre-stressing it. This has implications for the construction of the greenhouse roof. Research carried out by Delft University of Technology shows that it may be possible to use multi-layer thin glass as sandwich panels.”

Energy savings in phalaenopsis

The research team used this data to calculate how much energy the new material could save in the vegetative and flowering phases of a phalaenopsis crop. The choice of this crop was no coincidence. “Multi-layer glass is particularly interesting for crops grown at high temperatures, such as orchid or bromelia. In the future, the business case will be easiest to make for today’s greenhouses with double glazing or polycarbonate,” the scientist says.
An initial calculation shows that a quadruple-layer sheet of ultra-thin glass with AR treatment can save 20-25% heat in the two growth phases of phalaenopsis because of its high insulation value. Another comparison is that of double layer ultra-thin glass and polycarbonate sheets. They both insulate equally well but the first has much better light transmittance. This can cut the electricity bill in the warm cultivation phase by up to 20% as less lighting is needed. The effects on relative humidity and the use of conventional lighting are already factored into these calculations.

Pros and cons

The research concludes that ultra-thin glass definitely has potential, although not for a good while yet. There are plenty of benefits: the high light transmittance of AR-treated glass combined with the low weight, the bending strength, the high impact strength, the low transport costs and the potential for energy savings. But there are almost as many challenges, such as the high price of the material, the limited dimensions and the lack of rigidity.

Whether this material has potential for greenhouse horticulture depends on a number of things, Hemming believes. “If energy prices rise, it will be more worthwhile pursuing concepts like these,” she says. “The price of the material itself is also an issue. The project team expects it to fall as the product becomes more developed. The greenhouse horticulture market on its own is small, but sectors like the construction industry are taking an interest in it for architectural applications. For this material to break into our market, it will need to be available in larger dimensions. We won’t be able to scale up our research any further until that happens.”

Summary

Ultra-thin glass with a thickness of just 0.55 mm is already used on mobile phones and tablets. Researchers are exploring whether it could be an alternative to the conventional 4 mm glass used in greenhouses. The material is very lightweight and allows plenty of light through even in three or four layers, which boost its insulating properties. But first the price needs to come down and the glass needs to be available in larger dimensions. Once that happens, it could have potential for phalaenopsis or other warm crops.